You want to make sure they don't have chemical fertilizer in use. I have had people claim years ago that there shouldn't have been sheep or goats(can't remember which) On the property in 8 years however I have spoken with ranchers and they told me thats not something they'd worry about...
People are HUGE on using fertilizer anymore, so that is the main thing I would ask about. Ask how the hay is kept after it is cut. Is it kept in a dry barn? Is it left outside? You want something that is stored in dry area. What is the hay mixed with, if anything? Can you see the hay, and check the hay for mold or to insure it is NOT wet in the center of the bale?
We store our hay in the barn, and while the barn isn't the prettiest it is kept out of the rain, it's dried the proper time, and placed in the barn as soon as it's baled so it doesn't get wet and mold. The only fertizler we use is manure, nothing chemical here.
With anything though they can do their best to make sure THEY don't use chemicals, however it won't stop the neighboring farmer or farmer up the road from using it.